1. Field of the Invention
The present invention relates to a technique for controlling wavelength characteristics of optical transmission powers caused in wavelength division multiplexed signal light to be transmitted through an optical transmission path, and particularly to a method for compensating the wavelength characteristics of optical transmission powers making use of Raman amplification, as well as to a wavelength division multiplexing optical communication system and an optical amplifier to which the controlling method is applied.
2. Related Art
Wavelength division multiplexing (WDM) optical transmission system, such as making use of a wide gain band of an Erbium doped fiber amplifier, is capable of increasing a communication capacity by transmitting an optical signal including a plurality of wavelengths through a single optical fiber. This type of WDM optical communication system has such advantages that the system can be inexpensively introduced by utilizing existing optical fibers, and its transmission path is made to be bit-rate free by utilizing such as optical amplifier thereby facilitating future upgrade.
To obtain required transmission characteristics in a WDM optical communication system, it is necessary to restrict dispersions of light powers between channels to 1.0 dB or less at respective optical repeating stages. This is because an upper limit of light power is restricted by nonlinear effect of a transmission path, and a lower limit thereof is restricted by a receiving S/N due to spontaneous emission light of an optical amplifier. As such, it is required to diminish wavelength loss characteristics such as of transmission path and dispersion compensation fiber constituting a WDM optical communication system. Particularly, for an optical amplifier which collectively amplifies multi-wavelength optical signals and serves as a key component of a system, it is strongly desired that output deviations among channels be low in addition to that the optical amplifier transmits a predetermined output level with a low noise characteristic.
In an actual WDM optical communication system, there are caused wavelength characteristics of optical transmission powers among respective channels, such as due to the following matters:
{circle around (1)} wavelength loss characteristic of transmission path, due to Rayleigh scattering;
{circle around (2)} wavelength loss characteristic of dispersion compensator;
{circle around (3)} wavelength loss characteristic of transmission path due to induced Raman scattering;
{circle around (4)} wavelength characteristic of gain of optical amplifier; and
{circle around (5)} temperature characteristics of transmission path, of dispersion compensator and of optical amplifier.
Concretely, such as in case of adopting a 1.3 xcexcm zero-dispersion single mode fiber (SMF) of a length of 80 km provided that a wavelength band of signal light is between 1530 to 1560 nm, deviations of optical transmission powers caused due to the matters {circle around (1)} and {circle around (3)} are approximately 0.5 dB and approximately 1 dB, respectively. Deviation to be caused by the matter {circle around (2)} is approximately 0.5 dB when a general dispersion compensation fiber (DCF) is adopted as a dispersion compensator, while deviation to be caused by the matter {circle around (4)} is approximately 1 dB when a general Erbium doped optical fiber amplifier (EDFA) is adopted. Further, deviation to be caused by the matter {circle around (5)} can be evaluated as being approximately 0.3 dB when adopting the aforementioned optical devices.
The thus caused wavelength characteristics have different magnitudes depending on transmission conditions (such as number of channels, channel separations, input power, transmission path length). As such, it is necessary to newly or additionally apply a means capable of compensating the variably caused wavelength characteristics, to a WDM optical communication system. When wavelength characteristics of optical transmission powers among channels are to be compensated at each of optical amplifying and repeating stages provided in a WDM optical communication system, a width or magnitude of wavelength characteristic to be compensated at one optical amplifying and repeating stage is approximately 3 dB.
Conventionally, there has been proposed a method in which such as variable gain equalizers capable of varying wavelength loss characteristics are newly applied to an optical communication system, as a scheme for compensating wavelength characteristics of optical transmission powers as described above. According to such a method, optical transmission powers among channels are equalized by controlling wavelength loss characteristics of variable gain equalizers corresponding to respective wavelength characteristics of optical transmission powers caused in the system.
However, in the aforementioned conventional method for controlling wavelength characteristic of optical transmission powers, the optical transmission powers are equalized by giving losses to respective channels other than a channel having a minimum power in a manner matching to the minimum power channel. As such, there is a possibility that an optical S/N ratio is degraded and transmission characteristics are thereby deteriorated, as compared to the time such as before insertion of variable gain equalizers.
As one method for compensating wavelength characteristics of optical transmission powers while restraining loss to a smaller value, it is effective to utilize Raman amplification. This method is adapted to compensate wavelength characteristics of optical transmission powers, by preferentially amplifying channels of lower powers.
As techniques utilizing Raman amplification, there are known articles of, for example, S. Kinoshita et al., OECC, 10B2-3, July, 1997; and Emori et al. entitled xe2x80x9cA broadband dispersion compensating Raman amplifier pumped by multi-channel WDM laser diodesxe2x80x9d, Technical Report of IEICE, OCS98-58, pp. 7-12, 1998. The techniques described in these articles have contemplated realizing lower loss of dispersion compensation fiber and broader band of optical amplifier, by Raman amplifying a dispersion compensation fiber within an optical amplifier by utilizing a pump light source such as at 1,480 nm band. Further, in Japanese Unexamined Patent Publication No. 10-73852, there is described an optical amplifying transmission system which has contemplated realizing a broader band of signal transmission, making use of Raman amplification.
However, in these known techniques utilizing Raman amplification, Raman amplification is generated by supplying fixed pump light of which wavelength band and power have been previously set, such as to a dispersion compensation fiber. Thus, when fluctuation has occurred in wavelength characteristics of optical transmission powers such as caused in a transmission path, it is difficult to flexibly change a gain wavelength characteristic of Raman amplification corresponding to such fluctuation. In a WDM optical communication system, since it is considered that such as the number of channels of WDM signal light to be transmitted and wavelengths to be used are changed variously, and that wavelength characteristics of optical transmission powers fluctuate, it is accordingly desired to flexibly compensate wavelength characteristics corresponding to such fluctuation.
The present invention has been carried out in view of the points as described above, and it is therefore an object of the present invention to provide a method for controlling wavelength characteristics of optical transmission powers utilizing Raman amplification, in which wavelength characteristics of optical transmission powers are automatically compensated without giving loss to each channel light, and transmission characteristics are improved by compensating a loss of transmission path making use of Raman amplification, and a wavelength division multiplexing optical communication system and an optical amplifier adopting the method.
To achieve the above object, the present invention provides a method for controlling wavelength characteristics of optical transmission powers caused in WDM signal light transmitted through an optical transmission path by Raman amplification, comprising: a Raman amplification generating step for supplying Raman pump light at a wavelength band set corresponding to the wavelength characteristics of optical transmission powers, to a Raman amplifying medium forming at least a part of the optical transmission path, so as to generate Raman amplification having gain wavelength characteristics capable of compensating the wavelength characteristics of optical transmission powers, for WDM signal light propagated through the Raman amplifying medium; a wavelength characteristic monitoring step for monitoring wavelength characteristics of optical transmission powers, for WDM signal light passed through the Raman amplifying medium; and a Raman amplification controlling step for adjusting a generating condition of the Raman pump light based on a result of the wavelength characteristic monitoring step, to thereby control the gain wavelength characteristics of Raman amplification such that the wavelength characteristics of optical transmission powers are flattened.
According to such a wavelength characteristic controlling method, the WDM signal light, transmitted through the optical transmission path and thereby caused with the wavelength characteristics of optical transmission powers (tilt), is Raman amplified by passing through the Raman amplifying medium supplied with the Raman pump light by the Raman amplification generating step. In this Raman amplification, the wavelength band of the Raman pump light is set to have the gain wavelength characteristics capable of compensating the wavelength characteristics of optical transmission powers, so that dispersions of optical transmission powers in the WDM signal light after Raman amplification are flattened. For this WDM signal light, the wavelength characteristics of optical transmission powers are monitored at the wavelength characteristic monitoring step, to thereby monitor as to whether the compensation of wavelength characteristics at the Raman amplification generating step is being effectively performed. Then, at the Raman amplification controlling step, the monitored result of the wavelength characteristic monitoring step is fed back so that the generating condition of Raman pump light is adjusted (for example, power of Raman pump light or controlling temperature of a pump light source is adjusted) such that the wavelength characteristics of optical transmission powers are flattened, to thereby control the gain wavelength characteristics of Raman amplification.
In this way, for WDM signal light caused with dispersions in its optical transmission powers, the wavelength characteristics of optical transmission powers thereof can be compensated by applying the present tilt compensation making use of the gain wavelength characteristics of Raman amplification basically without any losses, since an optical element to be newly inserted in the transmission path is only a pump light multiplexer. In addition, the wavelength characteristics of optical transmission powers after Raman amplification are monitored to thereby control the gain wavelength characteristics of Raman amplification. Thus, even when the number of channels of WDM signal light to be transmitted and wavelengths to be used are changed variously such that wavelength characteristics of optical transmission powers are varied, there can be realized flexible compensation of wavelength characteristics corresponding to such variation.
In the above wavelength characteristic controlling method, the Raman amplification generating step may supply Raman pump lights at a plurality of wavelength bands to the Raman amplifying medium, and the Raman amplification controlling step may adjust proportions of Raman pump lights at the respective wavelength bands to thereby control gain wavelength characteristics of Raman amplification. Further, it is preferable that the Raman pump lights at the plurality of wavelength bands include Raman pump lights at such wavelength bands generating Raman amplifications having mutually different inclined directions of gain wavelength characteristics within a signal light band. As a specific adjusting method of the Raman pump lights, it is possible to fix a generating condition of the Raman pump light of at least one wavelength band among the Raman pump lights at the plurality of wavelength bands, and to adjust generating conditions of the Raman pump lights of the remaining wavelength bands.
In this way, the gain wavelength characteristics of Raman amplification are varied corresponding to the proportions of the pump lights at respective wavelength bands, so that the gain wavelength characteristics of Raman amplification can be controlled with excellent precision.
Moreover, the wavelength characteristic controlling method may further comprise: a power monitoring step for monitoring an output light power of the Raman amplifying medium; and in which the Raman amplification controlling step may adjust proportions of the Raman pump lights at the respective wavelength bands, based on a result of the wavelength characteristic monitoring step and based on a result of the power monitoring step, such that the output light power of the Raman amplifying medium is kept constant and the wavelength characteristics of optical transmission powers are flattened.
In this way, there can be obtained wavelength division multiplexed signal light having a constant light power and flattened wavelength characteristics.
Moreover, as a specific Raman amplification generating step of the wavelength characteristic controlling method, the Raman pump lights may be supplied, by utilizing the optical transmission path formed of a dispersion-shifted fiber as the Raman amplifying medium. Alternatively, the Raman pump lights may be supplied, by utilizing a dispersion compensation fiber provided on the optical transmission path as the Raman amplifying medium.
In this way, by utilizing a dispersion-shifted fiber or a dispersion compensation fiber having a relatively small mode field diameter as the Raman amplifying medium, there can be generated a required Raman amplification by a smaller power of Raman pump light.
The aforementioned method for controlling wavelength characteristics of optical transmission powers by Raman amplification, can be applied to various devices such as WDM optical communication system and optical amplifier to be described later, to thereby realize improvement of transmission characteristics of WDM signal light.